1. Field of the Invention
This invention relates to a telecentric illumination system which is capable of increasing the magnification changing area on the high magnification side of the telecentric illumination system.
2. Description of the Prior Art
For example, in an illumination system in a projection inspection device or the like, telecentric illumination has heretofore been adopted to minimize the influence of the in-focus error during measurement. To obtain a projection image which is always bright even if the magnification is changed, there has been proposed a variable magnification telecentric illumination system which is capable of changing the illumination area in accordance with a change in magnification of an objective lens as shown in Japanese Patent Application No. 68997/1980 (corresponding U.S. application Ser. No. 263,174, now U.S. Pat. No. 4,386,828) and Japanese Patent Application No. 93729/1981 by the applicant of this application. However, the magnification change ratio of the illumination system is limited and in reality, it is very small as compared with the magnification ratio from the very low magnification to the super-high magnification of a projection lens. Therefore, to secure the brightness at the super-high magnification, it is necessary to increase the magnification changing area on the high magnification side of the telecentric illumination system.
Heretofore, in a telecentric illumination system as shown in FIG. 1 of the accompanying drawings, it has usually been practised to add a convergent lens group L between a condenser lens L.sub.C and an object surface O as shown in FIG. 2 of the accompanying drawings and cause a light source image S' to be formed near the object O to be illuminated and thereby increase the brightness. In this method, however, conditions of telecentric illumination are destroyed and therefore, this method is not preferable for a measuring instrument. Also, if the light source image is brought too close to the object to be illuminated, there will occur numerous inconveniences that the filament is seen and that irregularity of illumination is caused. Therefore, the light source image must unavoidably be spaced to some degree from the object to be illuminated and, as shown in FIG. 2, the marginal light beam cannot be used effectively and the loss of quantity of light is great, and this is inefficient. Also, a projection lens is designed telecentrically, and particularly in the case of a lens having a long working distance, the effective diameter of the fore lens thereof is great and therefore, if an illumination greatly broken from telecentric illumination is effected, light which is not actually utilized for image formation will enter the lens, thus resulting in flare or ghost effects.
It is an object of the present invention to overcome the above-noted disadvantages and to provide a telecentric illumination system which is capable of increasing the magnification changing area on the high magnification side while effectively using the light source and light beam of the telecentric illumination system, thereby effecting bright illumination of higher magnification.
The telecentric illumination system according to the present invention has first and second convergent lens groups disposed with a spacing therebetween corresponding to the sum of the focal lengths thereof, and is inserted in the object side of a predetermined telecentric illumination device to achieve telecentric illumination of higher magnification than the illuminated condition by the predetermined telecentric illumination device.
The invention will become more fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings.